1. Technical Field
The invention relates to an electronic device, and more specifically, to a semiconductor memory device and an operating method thereof.
2. Related Art
A semiconductor memory device is a memory device implemented using a semiconductor, such as silicon (Si), germanium (Ge), gallium arsenide (GaAs), indium phosphide (InP), etc. The semiconductor memory device can be generally classified as a volatile memory device or a nonvolatile memory device.
A volatile memory device is a memory device in which data stored therein is erased when power supply is cut off. Volatile memory devices include a static random access memory (SRAM), a dynamic RAM (DRAM), a synchronous DRAM (SDRAM), etc. A nonvolatile memory device is a memory device in which data stored therein is retained even when power supply is cut off. Nonvolatile memory devices include a read only memory (ROM), a programmable ROM (PROM), an electrically programmable ROM (EPROM), an electrically erasable and programmable ROM (EEPROM), a flash memory, a phase-change RAM (PCRAM), a magnetic RAM (MRAM), a resistive RAM (ReRAM), a ferroelectric RAM (FeRAM), etc. Flash memories are widely classified into a NOR-type and a NAND-type.
A flash memory device may be classified as a 2-dimensional semiconductor device in which strings are formed on a semiconductor substrate in a horizontal direction, or a 3-dimensional semiconductor device in which strings are formed on a semiconductor substrate in a vertical direction.
The 3-dimensional semiconductor device is a memory device developed to overcome a limitation of a degree of integration of the 2-dimensional semiconductor device, and thus includes a plurality of strings formed in a vertical direction on the semiconductor substrate. The strings include a drain select transistor, memory cells and a source select transistor connected in series between a bit line and a source line.